Catalytic cracking processes are employed to produce gasoline and light distillate fractions from heavier hydrocarbon feed stocks. These cracking processes include fixed bed operations and fluid catalytic riser cracking processes. Preferred catalysts employed in such processes for the cracking of hydrocarbons boiling substantially above 600.degree. F. (316.degree. C.) comprise a crystalline aluminosilicate dispersed in a refractory metal oxide matrix such as disclosed in U.S. Pat. No. 3,140,249 and 3,140,253 to C. J. Plank and E. J. Rosinski.
The catalytic cracking processes of the prior art are normally conducted until the metal contaminants concentration on the catalyst reaches about 1500 parts per million (ppm) nickel equivalents (ppm nickel + 0.2 ppm vanadium) at which time the catalyst is replaced by fresh catalyst. The incremental replacement of the catalyst is expensive to insure that the maximum acceptable level of metal contaminants is not exceeded and a number of methods have been investigated for the purpose of lowering this high replacement cost. For example, it has been proposed to reduce the concentration of metals in the feed stock by pretreatment of the contaminated feed to lower the concentrations of metals to below about 1 ppm. A second suggestive method is to exclude by fractionation the heavier gas oils and residual fractions where the major concentrations of metal contaminants occur.
The proposed methods have been only partially successful and as the necessity for increasing the conversion of heavier feed stocks to the lower boiling product fractions becomes more important, it is evident that improved catalytic cracking processes which permit the charging of feed stocks containing relatively high concentrations of metals are needed. Also, it is desirable to reduce the rate of replacement of catalytic cracking catalysts attributable to the buildup of metal contaminants on the catalyst.